Method of producing prills

ABSTRACT

Method of producing prills includes providing a hollow body rotatable about a first axis, the body having a wall rotationally symmetrical around the first axis forming an interior space, the wall including nozzles; providing a second body disposed in the hollow body such that a gap exists between the hollow body and the second body; supplying liquid to the gap; generating jets of liquid from the nozzles radially outward by driving the rotational motion of the hollow body and/or second body around the first axis of rotation using a rotary drive unit; applying a reciprocal pressure excitation on the jets of liquid by moving the hollow body and/or second body along the first axis; and decoupling the rotations of the one of the hollow body and second body and a reciprocating drive-unit.

This application is a national stage filing under 35 U.S.C. 371 ofpending International Application No. PCTNL2021/050079, filed Feb. 5,2021, which claims priority to Netherlands Patent Application No.2024841, filed Feb. 5, 2020, the entirety of which applications areincorporated by reference herein.

The invention relates to a method of producing prills.

Prilling is a known process for converting a quantity of liquid, inparticular a quantity of a molten substance, into a plurality ofreasonably uniform spherical particles. It comprises two operations:firstly, generating liquid drops from the quantity of liquid andsecondly, solidifying the liquid drops individually by cooling as theyfall through a rising ambient air stream. Since there is noagglomeration, the size distribution of the drops determines that of theproduct. Plastic prills and washing powders are examples of resultingproducts.

By installing a droplet generator at the top of a prilling tower, whichis essentially a large cooling tower, heat is transferred from the dropsto the air as it falls down and solidifies, wherein the tower has to beof sufficient height for the particles to be strong enough not to breakon impact with the tower floor.

Typically two approaches for generating these droplets aredistinguished. The first approach uses a static prilling bucket, whichin its simplest form can be best compared to a shower head, whereinliquids are pushed through a stationary bucket comprising distributednozzles for generating jets of liquid. These jets break up after acertain distance thereby forming droplets. The second approach uses arotary prilling bucket. Rotary prilling buckets comprise distributednozzles in the side wall and/or bottom. By spinning the bucket at acertain rotational speed, centrifugal forces push the liquid through thenozzles, thereby generating the jets. Prilling apparatuses comprisingrotary prilling buckets tend to have a larger production capacity thanthose comprising stationary buckets.

An important step in the prilling process is generating droplets fromthe flow of liquid, in particular precisely and accurately controllingthe size of the droplets and thereby of the resulting prills. Thedistribution of the size of the prills (also known as the particle sizedistribution) generated using existing droplet dispensing methods istypically quite broad. Specifically, they typically produce“dust-particles”, which reduce the yield of the process, and tend tocontaminate the surrounding of the installation. On the other hand, theyalso typically produce droplets so large, that they are not sufficientlysolidified at the end of the process, i.e. when they reach the towerfloor. As a result, they break upon impact and may “glue” together (i.e.agglomerate) the spherical particles that are collected at the bottom ofthe tower. This leads to a further reduction of the yield of theprocess.

It is an object of the invention to alleviate at least a part of theabove mentioned problems. Specifically, it is an object of the inventionto increase the prilling yield.

Thereto, the invention provides a method of producing prills, comprisingthe steps of:

-   -   providing a hollow body arranged to rotate about a first axis of        rotation, the hollow body comprising a wall that is arranged        rotation symmetrically around the first axis, thereby enclosing        an interior space, the wall being provided with a plurality of        through-holes forming nozzles;    -   providing a second body being shaped to fit into the interior        space of the hollow body, nesting the second body inside the        hollow body, such that a gap is obtained between an inner        surface of the wall of the hollow body and an outer surface of        the second body;    -   supplying a flow of liquid, such as a molten substance, to the        gap, preferably through a liquid inlet that is in liquid        connection with the gap;    -   generating jets of liquid from the nozzles in at least a        radially outward direction with respect to the first axis by        driving the rotational motion of at least one of the hollow body        and second body around the first axis of rotation, preferably        using a rotary drive unit;    -   applying a reciprocal pressure excitation on the jets of liquid        by moving, preferably using a reciprocating drive unit, one of        the hollow body and second body with respect to the other of the        hollow body and second body along the first axis of rotation;        and    -   decoupling the rotations of the one of the hollow body and        second body and the reciprocating drive-unit.

By reciprocally driving, by means of the reciprocating drive-unit, oneof the hollow body and second body along the axial direction of thefirst axis, a changing pressure is applied to the liquid that is presentin the gap. It is noted that the second body is suspended, i.e. nested,in such a manner that a gap is formed that is large enough such that thehollow and second bodies do not contact each other, even when. Thesepressure pulses propagate to the jets of liquids that are generatedthrough the nozzles of the wall of the hollow body and result in an evenbreak up of the jet into droplets that are sized substantially moreevenly. In order to improve the accuracy with which the pressurevariation imposed on the liquid can be controlled, the rotations, atleast around an axis parallel to the first axis, of the one of thehollow body and second body and the reciprocating drive-unit aredecoupled. Hereby, the reciprocating drive-unit does not have to rotatewith the hollow body, which also enables a more simple and robustconstruction of the reciprocating drive-unit, nor does it have to beable to cope with any torsional forces, such as those propagating fromthe one of the hollow body and second body, that would otherwise beintroduced to the reciprocating drive-unit. For instance, even if onlythe hollow body would be driven, the liquid being present in the gapwould effectively act as a hydraulic coupling, thereby transferring atorque to the second body. Hence, decoupling the reciprocatingdrive-unit from these rotations and associated torques enables using astacked piezo element in the reciprocating drive unit. Since these areable to generate vibrations in a large band of frequencies with asufficiently large force-amplitude and can be controlled precisely,strong vibrations of small amplitudes can be generated at apredetermined frequency and transferred to the one of the hollow bodyand second body. This enables exciting the liquid that is in the gap byreciprocal pressure excitations having the predetermined frequency.Piezo elements can, however, be extremely sensitive to torsional loadingand thus easily damage.

The method enables creating a substantially uniform pressuredistribution in the liquid over substantially the full circumference ofthe interior space. It enables controlling the size of the droplets moreprecisely and accurately, thereby improving the performance of theprilling process in terms of particle/drop size distribution. Moreover,it allows for distributing the drops more uniformly across the width ofa prilling tower, thereby further increasing the yield of the prillingprocess. The method thus allows for creating droplets with lessvariation in size, when compared to a prilling method according to theprior art. The flow of liquid, such as a molten substance, is in theprilling process transformed to droplets, which can solidify over timethereby obtaining prills. It should be noted that the method, i.e. themethod of dispensing droplets, can also be applied to other processeswherein dispensing droplets with less variation in size is advantageous.

In a preferred embodiment, the step of decoupling the rotationscomprises providing a coupling mechanism between the reciprocatingdrive-unit and the one of the hollow body and second body; enabling, bymeans of the coupling mechanism, relative rotations, at least around anaxis parallel to the first axis, between the one of the hollow body andsecond body and the reciprocating drive-unit. A coupling mechanism, orcoupling unit, is arranged for enabling the relative rotations betweenthe one of the hollow body and second body and the reciprocatingdrive-unit in a reliable manner.

In a preferred embodiment the coupling mechanism comprises a firstrotational bearing unit and a second axis of rotation, wherein the stepof providing the coupling mechanism comprises coupling a lower end ofthe reciprocating drive unit to a first part of the first rotationalbearing unit and coupling the one of the hollow body and second body toa second part of the first rotational bearing unit, and wherein, duringthe step of applying a reciprocal pressure excitation, a lower end ofthe reciprocating-drive unit moves in a direction substantially parallelto the second axis between a first and second position; and wherein thestep of enabling relative rotations comprises rotating the second partof the rotational bearing unit with respect to a first part of the firstrotational bearing unit around the second axis of rotation. Thereciprocating-drive unit and the one of the hollow body and second bodyare thereby effectively coupled through the rotational bearing, suchthat substantially no damaging torsional loads are transferred to thereciprocating drive unit.

In a preferred embodiment, the provided hollow body is at leastpartially substantially cylindrically and/or conically shaped, whereinthe interior space is at least partially substantially cylindrically orconically shaped, and wherein the provided second body is shapedsubstantially similar to the interior space of the hollow body, suchthat a width of the obtained gap is substantially constant along anentire circumference of the second body. In such a hollow body asubstantially uniform pressure distribution can be created in the liquidover substantially the full circumference of the interior space, whichin turn improves the performance in terms of uniformly sized drops andalso allows to distribute them more uniformly across the width of aprilling tower, thereby increasing the yield of the prilling process.

It is then further preferred that the method further comprises the stepof coaxially arranging the hollow body, second body and the firstrotational bearing unit, such that the first and second axes of rotationcoincide and the hollow body, second body and the first rotationalbearing unit rotate around the same axis of rotation. Herebysubstantially only axial forces act during use on the first rotationalbearing unit, such that there is no need to transfer any other reactionforces to, for instance, a supporting frame of the apparatus and arobust and simple construction can be achieved.

The method, according to a preferred embodiment, further comprises thestep of controlling the reciprocating drive-unit, using a controller, tomove the one of the hollow body and second body with respect to theother of the hollow body and second body with a predefined frequency andamplitude of motion. The controlling action of the controller allowscustomizing the frequency and/or amplitude with which to drive thereciprocating drive unit such that the yield is further increased. Itwas, for instance, found that an operational frequency and/oroperational amplitude for driving the reciprocating driving unit can bedetermined, for instance, on the basis of the viscosity of the liquid,whereby the jet breaks up into droplets that are sized substantiallymore evenly and thereby increases yield.

In a preferred embodiment, the method comprises the step of coupling thereciprocating drive-unit to a frame assembly and suspending the couplingmechanism in the axial direction from the reciprocating drive-unit. Thisallows for the use of a reciprocating drive-unit that requires a certainamount of tensile pretension in order to operate accurately andreliably. It is further preferred that the method further comprises thesteps of arranging a second rotational bearing unit, which is arrangedto rotate around a third axis of rotation, between the reciprocatingdrive-unit and the frame, wherein the third axis of rotation issubstantially parallel, and preferably coincides, with the second axesof rotation. Hereby, the reciprocating drive-unit is arranged betweentwo bearing units and is free to rotate around its axis. Even in casethe first bearing unit would (partly) fail, the second bearing unit isstill able to prevent that the reciprocating drive-unit has to take upan excess torque originating from the one of the hollow body and secondbody.

In a preferred embodiment the method comprises the step of blocking, bymeans of a rotational blocking mechanism, substantially any rotation ofreciprocating drive-unit around the second axis of rotation. Thecoupling mechanism preferably comprises the rotational blockingmechanism. Due to, for instance, a small amount of friction in thecoupling mechanism, a relatively small torque might still be able topass through the coupling mechanism. By blocking the rotation using theprovided blocking mechanism, this torque is taken up by the blockingmechanism and thereby diverted around the reciprocating drive-unit. Itis preferred that blocking pin is provided, preferably in the blockingmechanism, for blocking the rotation (with respect to the frame), asthis allows for a simple and reliable means of blocking a rotation.

A preferred embodiment further comprises the step of providing a stackedpiezo-electric element in the reciprocating drive-unit, and contractingand/or expanding the stacked piezo-electric element in a directionsubstantially parallel to the first axis for moving the one of thehollow body and second body with respect to the other of the hollow bodyand second body. Stacked piezo-electric element can deliver the forcesrequired for propagating the pressure variation, or pressure pulses, tothe jets, such that they break up in more equally sized droplets. Inaddition the stacked piezo-electric element can be driven accurately interms of frequency and amplitude, even at the force levels required.

The step of preloading the reciprocating drive-unit using a biasingmechanism is further comprised in a preferred embodiment of the method.Certain types of actuators require a certain amount of pre-loading tofunction properly. As non-limiting example, stacked piezo-electricelements are available that require a minimum predefined amount oftensile preload to be able to reliably function. It is for instancepreferred that the step of preloading the reciprocating drive-unit usinga biasing mechanism comprises suspending the coupling mechanism and theone of the hollow body and second body from the reciprocating drive-unitfor applying a tensile preload to the reciprocating drive-unit. Herebygravity itself acts as the pre-load applied to the reciprocatingdrive-unit, such that a simple and robust biasing mechanism can beobtained.

Preferably, the method further comprises providing a shaft-assemblycomprising a first and second shaft, and arranging the second shaftbetween the coupling mechanism and the one of the hollow body and secondbody and arranging the first shaft between the rotary drive unit and theother of the hollow body and second body. As the second body is arrangedwithin the interior space of the hollow body and the bodies need todriven in a different manner, the shaft-assembly is arranged such thatthe second shaft is arranged between the coupling mechanism and the oneof the hollow body and second body and allows for driving the one of thehollow body and second body in a reciprocating manner; and such that thefirst shaft is arranged between the rotary drive unit and the other ofthe hollow body and second body to allow to rotate at least the other ofthe hollow body and second body, thereby subjecting the liquid in thegap to the centrifugal forces. It is preferred that the method furthercomprising the step of arranging the first and second shafts coaxially,as hereby a compact shaft assembly is obtained that is suited for thedescribed purpose. Alternatively, or additionally, the method,preferably, comprises the step of arranging the first shaft to at leastpartially enclose the second shaft in the radial direction, or arrangingthe second shaft to at least partially enclose the first shaft in theradial direction. Hereby, one can also obtain a compact shaft-assembly,wherein the inner shaft is also shielded by the outer shaft.

It is preferred that, alternatively, or additionally, the step ofproviding a shaft-assembly further comprises providing a third bearingsystem, arranging the third bearing system in between the first andsecond shafts, wherein the third bearing system comprises at least alinear bearing member; wherein the method further comprises the step ofmoving the second shaft with respect to the first shaft in the axialdirection while moving, using a reciprocating drive unit, one of thehollow body and second body with respect to the other of the hollow bodyand second body along the first axis of rotation; these are preferablythe only relative movements that the third bearing system allows. Thethird bearing system thereby enables a smooth operation with reducedfriction. In addition, by coupling the first and second shafts throughthe third bearing system, forces exerted on the second shaft are, apartfrom the torque around the shaft and the axial forces, transferred tothe first shaft instead of the coupling mechanism.

In a preferred embodiment, the method further comprises the steps ofarranging a rotational transfer mechanism between the hollow body andsecond body; coupling the rotational motion of the hollow body andsecond body around the first axis by means of a rotational transfermechanism. Hereby the rotational motion that is transferred from therotary drive to the hollow body is transferred to the second body aswell. As both bodies thereby rotate with the same rotational speed,shear effects in the liquid are reduced, these shear effects would, forinstance, be present due to a difference in speed of the hollow body andsecond body and having a speed reducing effect on the liquid. Hence, bycoupling the rotational motion of the hollow body and second body, thecentrifugal forces acting on the liquid in the gap, and thereby thepressure of the liquid acting on the inner surface of the wall of thehollow body, can thus be controlled more accurately.

Preferably, the provided rotational transfer mechanism comprises a maleconnecter arranged at a nonzero radial distance from the first axis ontoone of the hollow and second body and a female connector arranged to theother of the hollow and second body and wherein the female connecter isarranged for slidably receiving the male connector, for coupling therotational motion of the hollow and second body while allowing for therelative movement between the hollow and second body in the axialdirection. Hereby, a simple and robust coupling of the rotations can beobtained.

In a preferred embodiment, the method comprises the step of supplying aflow of liquid to the gap comprises supplying the liquid through theliquid inlet that debouches in the second body and subsequently throughat least one through hole of the second body that forms a liquidconnection between the gap and the liquid inlet. In use the liquid ishereby supplied to the gap located between the hollow and second bodies.It is then preferred that the method further comprising the steps of:arranging a primary through hole that runs substantially parallel to thefirst axis and debouches in a lower section of the hollow body;supplying the liquid through the primary through hole to the lowersection of the hollow body. Hereby, the supply of liquid to the gap issubstantially evenly over the circumference of the gap in radialdirection, whereby it is then also preferred if the primary through holeis arranged substantially centrally in the second body. Alternatively,or additionally, the method can further comprise the steps of:arranging, in the circumference of the second body, secondary throughholes that run at least outwardly in the radial direction as seen fromthe first axis; supplying the liquid through the secondary through holesto the gap. Hereby, the supply of liquid can be distributed over thegap, such that a more evenly distribution over the circumference of thegap in the axial direction is obtained. It is then preferred that thestep of arranging the secondary through holes comprises arranging thesecondary through holes of the second body, as seen along the radialdirection, at a nonzero distance of the nozzles of the hollow body. Byarranging the holes and outlets such that they substantially do notoverlap, a more even pressure distribution can be obtained over theinner wall of the hollow body. A more even pressure distribution at thewall leads to more even (liquid) conditions at the nozzles arranged inthe wall. Hereby, the conditions of the jets are more evenly and thusthe droplets are formed more evenly over the different nozzles.

In a preferred embodiment, the method comprises the steps of: arrangingsubstantially fin shaped members in the through-hole of the second body;and urging, by means of the fin shaped members, the flow of liquid torotate with the rotation of the second body. Fin shaped members can beused for imparting the rotational motion of the hollow and/or secondbody onto the flow of liquid entering from the liquid inlet. Thefin-shaped members preferably protrude inwardly from a circumferentialwall that delimits the through hole. It is further preferred that thefin-shaped members protrude substantially radially inwardly and/orwherein they run over substantially the full height of the through holeat at least the location of the circumferential wall. Hereby, the liquidflow can effectively be forced into a spinning motion by the rotation,thereby forming a stable vortex of the liquid inside the hollow orsecondary body.

In a preferred embodiment, the hollow body and second body are hollowconical frustums and wherein the step of applying a reciprocal pressureexcitation comprises reciprocally varying the width of the gap betweenthe hollow body and second body. A bucket shaped body can, for instance,be shaped on the outside like a truncated cone, i.e. a conical frustum.The hollow body and/or second body comprise preferably a substantiallysimilarly shaped interior space. Due to the bucket shape (i.e. truncatedcone or conical frustum), the bodies are substantially symmetrical, suchthat, by arranging the bodies substantially co-axial, an even gap isobtained between the outer surface of the second body and the innersurface of the wall of the hollow body. By reciprocally driving the oneof the hollow body and second body, the width of the gap is reciprocallyvaried, such that pressure pulses can be introduced in the liquid thatcan be present in the gap.

In a preferred embodiment, wherein the jets of liquid break up intodroplets, the method further comprising the steps of: generating a flowof a cooling fluid; at least partially solidifying the dispenseddroplets by cooling as they move through the generated flow of coolingfluid. Hereby, the droplets of liquid, preferably a molten substance,solidify due to flow of cooling fluid. A cooling fluid can, dependingon, for instance, the properties of the liquid, either be a coolingliquid or a cooling gas. In addition, the direction of the flow ofcooling fluid can either be substantially along the direction of thefalling droplets (e.g. substantially vertically downwardly in thedirection of gravity), but can also be substantially opposite to thedirection of the falling droplets (e.g. substantially verticallyupwardly in the opposite direction of gravity). By varying theseparameters of the flow of the cooling fluid, the time (and/or distance)with which the droplets at least partially solidify can be tuned to thedesired specification. For instance, it can be ensured that, even forvery small prills, the prills fall down to a bottom of a suitablecooling tower and that, when the prills arrive at the bottom of thecooling tower, they are at least solidified to such an extend thatagglomeration of the prills is significantly reduced, or evensubstantially prevented.

The present invention is further illustrated by the following figures,which show preferred embodiments of the method, wherein a dropletdispensing apparatus is used for generating prills from a flow ofliquid. The figures are not intended to limit the scope of the inventionin any way, wherein:

FIG. 1 schematically shows a 3D perspective view of a droplet dispensingapparatus for producing prills that is used for performing an embodimentof the method according to invention.

FIG. 2 schematically shows a cross-sectional view of the dropletdispensing apparatus of FIG. 1 in a first plane.

FIG. 3 schematically shows the cross-sectional view of the dropletdispensing apparatus in the first plane zoomed in on a top section ofthe apparatus.

FIG. 4 schematically shows the cross-sectional view of the dropletdispensing apparatus in the first plane zoomed in on a bottom section ofthe apparatus.

FIG. 5 schematically shows a cross-sectional view of the dropletdispensing apparatus of FIG. 1 in a second plane.

FIG. 6 schematically shows the cross-sectional view of the dropletdispensing apparatus in the second plane zoomed in on a top section ofthe apparatus.

FIG. 7 schematically shows the cross-sectional view of the dropletdispensing apparatus in the second plane zoomed in on a bottom sectionof the apparatus.

FIG. 8 schematically shows a preferred embodiment of a reciprocaldriving-unit and a coupling mechanism for use in the droplet dispensingapparatus.

FIG. 9 shows a picture of experimental results of traditional dropletdispensing method.

FIG. 10 shows a picture of experimental results of droplet dispensingmethod according to an embodiment of the invention.

FIG. 11 schematically shows two different types of nozzles arranged inthe circumferential wall of the hollow body.

FIG. 1 schematically shows a 3D perspective view of a droplet dispensingapparatus that is used for performing an embodiment of the methodaccording to invention. The droplet dispensing apparatus 1 comprises alower rotating assembly 2 that comprises the hollow and second bodies21, 22. A rotary drive unit 3 is arranged for driving the rotation ofthe lower rotating assembly 2, a reciprocating drive unit 4 forreciprocally driving the second body 22 along the axis of rotation I anda coupling mechanism 8 for decoupling rotations from the reciprocatingdrive unit 4. The apparatus can further comprise a stationary frameassembly 5 that comprise, for instance, a mounting bracket 51 formounting the apparatus in a suitable cooling tower, i.e. prilling tower(not shown). Also, cylinder 52 resembles the size of the opening throughwhich the apparatus typically needs to be inserted for mounting it in aprilling tower. An inlet piping system 6 is provided for supplying aliquid to the lower rotating assembly 2 of the apparatus 1, as isdescribed in more detail below. With reference to FIGS. 1-8 and 11 theworkings of the embodiment of the apparatus 1 will be described below inmore detail.

FIGS. 2-4 schematically show the cross-sectional view of the dropletdispensing apparatus 1 for producing prills of FIG. 1 in a first plane.FIGS. 5-7 schematically show the cross-sectional view of the dropletdispensing apparatus of FIG. 1 in a second plane which is substantiallyperpendicular to the first plane. Lower rotating assembly 2 comprises arotating hollow body 21, wherein a rotating second body 22 is arranged.Hollow body 21 and second body 22 are shaped such the second body 22 isshaped (at least on its outside) to fit into, and substantially match inshape with, the inner space 211 of the hollow body 21, thereby forming agap 23 between the outer surface of the second body 22 and the innersurface of the circumferential wall of the hollow body 21. The hollowbody 21 and second body 22 are preferably substantially bucket-shaped(i.e. are formed as hollow conical frustums) and are arranged such that,once installed in a prilling tower, the top sections of the bodies 21,22 have a larger dimension (e.g. diameter) than the bottom sections ofthe bodies 21, 22. This aids in a more evenly distribution of thedroplets throughout the prilling tower.

The second body 22 can comprise an opening 221 at its bottom in additionto a plurality of smaller through holes 222 that can be arranged over anumber of rows of through holes 222 that can be arranged at different(angular) locations in the circumferential wall of the second body 22,as seen around the axis of rotation I. These rows of through holes 222run can run over substantially the full height of the second body 22.

A central inlet 24 directs, in use, a flow of liquid to an interiorspace of the second body 22. The central inlet 24 can be fitted with aplurality of flow directing elements 241, 242 that aid in directing theflow towards the interior space of the second member 22 and/or in thedirection of rotation. After this, it flows through opening 221 and/orthe plurality of through holes 222 to the gap 23. A plurality of throughholes, also referred to as nozzles 91, 92, is arranged in thecircumferential wall 212 of the hollow body 21. In use, the hollow body21 spins around the rotational axis I, such that any liquid held in thegap 23 is exposed to the centrifugal forces originating from thisspinning, thereby creating a pressure in the liquid and which is forcedout the plurality of nozzles 91, 92, thus forming jets of liquid 901,911 (see FIGS. 9 and 10 ) that are directed at least partially in theradial outward direction with respect to the rotational axis I. As thewidth of gap 23 can be reciprocally varied by driving the reciprocatingdrive unit, as is described in more detail below, pressure pulsationscan be introduced to the liquid that is present in the gap 23. Thesepulsations will propagate to the jets shooting from the nozzles 91, 92.By tuning the frequency and the amplitude with which the width of thegap 23 is varied, pressure pulsations can be obtained that lead to afast break up of the jets into droplets, wherein substantially equallysized droplets are obtained, such that a spread in droplet-size issignificantly reduced.

Nozzles 91, 92 (see FIG. 11 ) can be arranged in different manners inthe circumferential wall 212. For instance, first nozzles 91, secondnozzles 92 or any combination of these and other types of nozzles can bearranged in the circumferential wall 212. The first nozzle 91 isarranged as a through hole that is substantially perpendicular to theouter and/or inner surface of circumferential wall 212. The secondnozzle 92 is arranged as a through hole that, once the dropletdispensing apparatus is installed in a prilling tower, runssubstantially horizontal, i.e. substantially perpendicular to the axisof rotation I. Alternatively, a recession 93 can be arranged in theouter surface of circumferential wall 212 of the hollow body 21, suchthat the though hole of the second nozzle 92 debouches in the recession93, wherein the surface of the recession 93 is substantiallyperpendicular to the through hole of the second nozzle 92.

The second body 22 can further comprise a number of fin-shaped members223 that extend from a centre-axis connecting body 224 in asubstantially radial direction towards the circumferential wall 225 ofthe second body 22. These fin-shaped members 223 force the liquid thatenters the interior space of the second body 22 to rotate with thesecond body 22. Furthermore, additional flow direction elements 226 canbe arranged at a top section of the fin-shaped members 223 to helpdistribute the liquid throughout the second body 22. Hereby, a stableand substantially constant vortex of the rotating liquid can be obtainedin the second and hollow bodies, thereby generating more constantprocess conditions at the nozzles 91, 92 and thus a better control ofthe process. In this embodiment, the second body 22 is connected at itstop section 226 to the top section 213 of the hollow body 21. The hollowbody 21 itself is driven by the rotary drive unit 3. An outer shaft 71is provided that is connected to the rotary drive unit on a first end711 and to the top second 213 of the hollow body at the second end 712.The outer shaft 71 can be connected to a stationary frame assembly 5 bymeans of rotary bearings 74. Stationary frame assembly comprises a framemounting bracket 51 for installing, or arranging, the droplet generatingapparatus 1 in a prilling tower.

The second body 22 is connected to an inner shaft 72 using thecentre-axis connecting body 224 that is arranged to receive and coupleto a lower portion 721 of the inner shaft 72. Inner shaft 71 is for themost part enclosed by the outer shaft 72 and supported by a number ofsliding bearings 73, such that the inner shaft 71 is, preferably only,movable with respect to the outer shaft 72 in a direction along the axisof rotation I and in the rotational direction around axis of rotation I.To shield the sliding bearings 73 and the space between the inner andouter shafts 71, 72 from dust and/or liquid contamination, a flexibleshaft cover 75 is arranged between the bottom section 712 of the outershaft 71 and the centre-axis connecting body 224.

FIG. 3 schematically shows the cross-sectional view of the dropletdispensing apparatus in the first plane zoomed in on a top section ofthe apparatus 1. Rotary drive unit 3 is provided for driving the outershaft 71 using, in the current embodiment, a second pulley 34 that canbe directly connected to the first end 711 of the outer shaft 71. Therotary drive unit 3 can comprise (electro-) motor 31 that drives a firstpulley 32 and wherein the first and second pulleys 32, 34 are coupled bymeans of a drive belt 33 that transfers the rotary motion from the motor31 to the outer shaft 71. Note however, that any other suitablerotational transfer, or gearing, mechanism can be used for this. Theouter shaft 71 is coupled, through rotational bearings 74 to a shaftholding frame 54, which is formed by a tubular member connecting andholding the rotational bearings 74, and wherein the shaft holding frame54 is in turn connect to a frame base member 53 that also comprises themounting bracket 51.

The inner shaft 72, which for the largest part enclose by the outershaft 71, extends at its upper end 721 from the first end 711 of theouter shaft. The upper end 721 is received by an output shaft 81 of thecoupling mechanism 8. The coupling mechanism 8, which comprises arotational bearing 82, takes up the rotational motion of the inner shaft72, thereby shielding the reciprocating drive-unit 4 from any torsionalforces that could potentially damage the vibrating element 41, which ispreferably a stacked-piezo element. Stacked-piezo elements are able togenerate vibrations in a large band of frequencies, with a sufficientlylarge force-amplitude and be controlled precisely, such that smallamplitudes of vibration can be obtained.

To secure the vibrating element 41 accordingly, the element 41 is heldbetween a lower 42 and upper connecting member 43. The vibrating element41 is directly connected and held to the upper connecting member 43. Thelower connecting member 42 is directly coupled to an upper section 83 ofthe coupling mechanism 8. The upper connecting member 43 is held by asecondary coupling mechanism 84 that also comprises a rotational bearing85. Hereby, the reciprocating drive-unit 4 is unconstrained in itsrotation around axis of rotation I, such that even if minor torsionalforces are transferred through the coupling mechanism 8, the vibrationelement 41 is virtually isolated from any potentially damaging torsionalforces that could potentially transfer from the inner shaft 72. Tofurther aid in this, coupling mechanism 8 comprises a blocking pin 86that transfers resulting torsional forces to a frame suspension member55.

The secondary coupling mechanism 84 is directly connected through itsupper, stationary, section 86 to the suspension member 55. Hereby, thereciprocating drive unit 4, coupling mechanism 8, inner shaft 72 andsecond body 22 are all suspended from the suspension member 55. Theaxial forces from these parts are thus transferred through the vibratingelement 41, which thereby has a preload applied to it. These suspendedparts 8, 72, 22 thus effectively form a biasing mechanism for thevibrating element 41. Suspension member 55 is part of the stationaryframe assembly 5.

FIG. 5 , which shows a cross-sectional view taken in a planesubstantially perpendicular to the plane of FIGS. 2-4 , shows the liquidinlet section 6 that comprises an assembly of tubular members and isarranged to connect on its first end 61 to a liquid feed system and onits second end 62 it debouches in the central inlet 24. Through astationary first section 244 of the central inlet 24, the liquid isarranged to flow to a second section 244 of the central inlet 24,wherein the second section 242 rotates with the hollow body 21.

In use, the hollow body 21 is driven by the rotary drive unit 3 torotate along the axis of rotation I, as described above. Liquid is fedto the second body 22 through the liquid inlet section 6, such that theliquid reaches the gap 23 through the second body 21 that comprises anumber of openings 221, 222. The reciprocating drive unit 4 is in turnused to vary the width of gap 23. In the current embodiment this isachieved by driving the vibrating element 41 that, through the couplingmechanism 8 and inner shaft 72, transfers the reciprocating motion alongthe axis of rotation I to the second body 22. By controlling thevibrating element 41 according to a predefined frequency and amplitude,pressure pulsations are introduced to liquid held in the hollow body 21.The combination of the centrifugal forces due to the rotation and thepressure pulsations introduced in the liquid, allows jets to formthrough nozzles 91, 92 that break up in individual droplets, wherein theindividual droplets have only a small variation in size (when comparedto a traditional rotational droplet generating apparatus), such thatthey can be considered to be substantially evenly sized.

Results from an experimental setup using such a device are show in inFIG. 10 , whereas results from an experimental setup of a traditionalrotational droplet generating apparatus are shown in FIG. 9 . Thephoto's show a hollow body 121 comprising both first and second nozzles91, 92. The second nozzles 92 are arranged to debouche in a recession 93arranged in the outer surface of the hollow body 121. In FIG. 9 anactual jet of liquid 901 can be seen leaving the nozzles 91, 92, whichonly after a certain distance break up into a series of differentlysized droplets 902. The droplets 902 have a large distribution in size,as the jets breaks up in larger primary droplets and smaller secondary,or satellite, droplets. As further downstream a number of differentdroplets might recombine to form even larger droplets, a large spread indroplet size is obtained.

In FIG. 10 , the liquid in the hollow body 22 is excited by pressurepulsations with a predefined frequency and amplitude. It is clear thatthe jets 911 start to break up practically immediately after it exitsfrom the nozzles 91, 92 and that the resulting droplets 912 are muchmore similar in size when compared to droplets 911. In addition, theindividual droplets 912 can be seen to spread out in more regularintervals, thereby leading to less merging of droplets. In theexperimental setup water/glycerine mixtures having different viscositieshave been tested. In the first test, water with a viscosity of 1 mPa*swas used, wherein it was found that excellent results could be obtained(in terms of a narrow distribution in droplet size) by rotating thebucket such that the water velocity in the resulting jets is 1.5 m/s andby introducing a pressure pulsations caused by introducing vibrations atapproximately 280 Hz and with an amplitude 20 μm. When using a somewhatmore viscous mixture of 4 mPa*s, a set of substantially ideal conditionswas, for instance, found when velocities of the jet of liquid are 1.3m/s and by introducing a pressure pulsations caused by introducingvibrations at approximately 240 Hz and with an amplitude 35 μm. Whenusing a more extreme mixture having a viscosity of 35 mPa*s, excellentresults were, for instance, obtained for velocities of 1.15 m/s and byintroducing the vibrations at approximately 190 Hz and with an amplitude35 μm.

FIG. 8 furthermore shows an alternative embodiment of the couplingmechanism 108 and an alternative embodiment of the secondary couplingmechanism 1084, wherein all the other components are equal to theembodiment shown in FIGS. 1-7 . Coupling mechanism 108 comprises tworotational bearings 1082, in particular spherical roller thrustbearings, which are preferably substantially equal. The bearings 1082are arranged such that the first ends 1086 are arranged adjacent to oneand another, such that the bearings only allow for the rotational motionof the output shaft 82 with respect to housing unit 187 of the couplingmechanism 108, such that a reliable coupling mechanism 108 with minimalplay in the axial direction is obtained. Play in the axial direction ofthe coupling mechanism 108 influences the transfer of vibrations fromthe vibrating element 41 to the second body 22 and would thereforenegatively affect the performance of the droplet generating apparatus 1.Also, spherical roller thrust bearings are highly suitable fortransferring high axial (i.e. trust) loads, such that a reliablecoupling mechanism 108 is obtained for transferring axial forces fromthe reciprocating driving unit 4 to the second body. Also the secondarycoupling mechanism 1084 comprises a similar arrangement of tworotational bearings 1085, in particular spherical roller thrustbearings.

The present invention is not limited to the embodiment shown, butextends also to other embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. A method of producing prills, comprisingthe steps of: providing a hollow body, the hollow body comprising a wallthat is arranged rotation symmetrically around the first axis, therebyenclosing an interior space, the wall being provided with a plurality ofthrough-holes forming nozzles; providing a second body being shaped tofit into the interior space of the hollow body, nesting the second bodyinside the hollow body, such that a gap is obtained between an innersurface of the wall of the hollow body and an outer surface of thesecond body, wherein at least one of the hollow body and the secondbody, and at least the hollow body, is arranged to rotate about a firstaxis of rotation; supplying a flow of liquid to the gap through a liquidinlet that is in liquid connection with the gap; generating jets ofliquid from the nozzles in at least a radially outward direction withrespect to the first axis by driving the rotational motion of at leastone of the hollow body and second body around the first axis of rotationusing a rotary drive unit; applying a reciprocal pressure excitation onthe jets of liquid by moving, using a reciprocating drive unit, one ofthe hollow body and second body with respect to the other of the hollowbody and second body along the first axis of rotation; providing acoupling mechanism between the reciprocating drive unit and the one ofthe hollow body and the second body; and decoupling the rotations of theone of the hollow body and second body and the reciprocating drive unit,which step comprises enabling, by means of the coupling mechanism,relative rotations between the one of the hollow body and the secondbody and the reciprocating drive unit.
 2. The method of producing prillsaccording to claim 1, wherein the coupling mechanism comprises a firstrotational bearing unit and a second axis of rotation, wherein the stepof providing the coupling mechanism comprises coupling a lower end ofthe reciprocating drive unit to a first part of the first rotationalbearing unit and coupling the one of the hollow body and second body toa second part of the first rotational bearing unit, and; wherein, duringthe step of applying a reciprocal pressure excitation, a lower end ofthe reciprocating-drive unit moves in a direction substantially parallelto the second axis between a first and second position; and wherein thestep of enabling relative rotations comprises rotating the second partof the rotational bearing unit with respect to a first part of the firstrotational bearing unit around the second axis of rotation.
 3. Themethod of producing prills according to claim 2, wherein the providedhollow body is at least partially substantially cylindrically and/orconically shaped, wherein the interior space is at least partiallysubstantially cylindrically or conically shaped, and wherein theprovided second body is shaped substantially similar to the interiorspace of the hollow body, such that a width of the obtained gap issubstantially constant along an entire circumference of the second body.4. The method of producing prills according to claim 3, comprising thestep of coaxially arranging the hollow body, second body and the firstrotational bearing unit, such that the first and second axes of rotationcoincide and the hollow body, second body and the first rotationalbearing unit rotate around the same axis of rotation.
 5. The method ofproducing prills according to claim 1, further comprising the step ofcontrolling the reciprocating drive-unit, using a controller, to movethe one of the hollow body and second body with respect to the other ofthe hollow body and second body with a predefined frequency andamplitude of motion.
 6. The method of producing prills according toclaim 1, further comprising: coupling the reciprocating drive-unit to aframe assembly and suspending the coupling mechanism in the axialdirection from the reciprocating drive-unit; and arranging a secondrotational bearing unit, which is arranged to rotate around a third axisof rotation, between the reciprocating drive-unit and the frame, whereinthe third axis of rotation is substantially parallel with the secondaxes of rotation.
 7. The method of producing prills according to claim2, further comprising: blocking, by means of a rotational blockingmechanism, substantially any rotation of reciprocating drive-unit aroundthe second axis of rotation; and providing the blocking mechanism with ablocking pin for blocking the rotation.
 8. The method of producingprills according to claim 1, further providing a stacked piezo-electricelement in the reciprocating drive-unit, and contracting and/orexpanding the stacked piezo-electric element in a directionsubstantially parallel to the first axis for moving the one of thehollow body and second body with respect to the other of the hollow bodyand second body.
 9. The method of producing prills according to claim 1,further comprising preloading the reciprocating drive-unit using abiasing mechanism; wherein the step of preloading the reciprocatingdrive-unit using a biasing mechanism comprises suspending the couplingmechanism and the one of the hollow body and second body from thereciprocating drive-unit for applying a tensile preload to thereciprocating drive-unit.
 10. The method of producing prills accordingto claim 1, further comprising providing a shaft-assembly comprising afirst and second shaft, and arranging the second shaft between thecoupling mechanism and the one of the hollow body and second body andarranging the first shaft between the rotary drive unit and the other ofthe hollow body and second body.
 11. The method of producing prillsaccording to claim 10, further comprising the step of arranging thefirst and second shafts coaxially.
 12. The method of producing prillsaccording to claim 10, comprising the step of arranging the first shaftto at least partially enclose the second shaft in the radial direction,or arranging the second shaft to at least partially enclose the firstshaft in the radial direction.
 13. The method of producing prillsaccording to claim 10, wherein the step of providing a shaft-assemblyfurther comprises providing a third bearing system, arranging the thirdbearing system in between the first and second shafts, wherein the thirdbearing system comprises at least a linear bearing member; wherein themethod further comprises the step of moving the second shaft withrespect to the first shaft in the axial direction while moving, using areciprocating drive unit, one of the hollow body and second body withrespect to the other of the hollow body and second body along the firstaxis of rotation.
 14. The method of producing prills according to claim1, further comprising the steps of: arranging a rotational transfermechanism between the hollow body and second body; and coupling therotational motion of the hollow body and second body around the firstaxis by means of a rotational transfer mechanism.
 15. The method ofproducing prills according to claim 1, wherein the step of supplying aflow of liquid to the gap comprises supplying the liquid through theliquid inlet that debouches in the second body and subsequently throughat least one through hole of the second body that forms a liquidconnection between the gap and the liquid inlet; the method furthercomprising: arranging a primary through hole that runs substantiallyparallel to the first axis and debouches in a lower section of thehollow body; and supplying the liquid through the primary through holeto the lower section of the hollow body.
 16. The method of producingprills according to claim 15, further comprising: arranging, in thecircumference of the second body, secondary through holes that run atleast outwardly in the radial direction as seen from the first axis, andsupplying the liquid through the secondary through holes to the gapwherein the step of arranging the secondary through holes comprisesarranging the secondary through holes of the second body, as seen alongthe radial direction, at a nonzero distance from the nozzles of thehollow body.
 17. The method of producing prills according to claim 15,comprising the steps of: arranging substantially fin shaped members inthe through-hole of the second body; and urging, by means of the finshaped members, the flow of liquid to rotate with the rotation of thesecond body.
 18. The method of producing prills according to claim 1,wherein the hollow body and second body are hollow conical frustums andwherein the step of applying a reciprocal pressure excitation comprisesreciprocally varying the width of the gap between the hollow body andsecond body.
 19. The method of producing prills according to claim 1,wherein the jets of liquid break up into droplets, the method furthercomprising the steps of: generating a flow of a cooling fluid; and atleast partially solidifying the dispensed droplets by cooling as theymove through the generated flow of cooling fluid.